An undisclosed injury has forced the ex-UFC champ to withdraw from "Strikeforce: Cormier vs. Mir," which takes place Nov. 3 at Chesapeake Energy Center in Oklahoma City, Okla., and airs live on Showtime and Showtime Extreme.

MMAjunkie.com (mmajunkie.com) confirmed the news on Tuesday night with a source close to the promotion following an initial report from news show Sports Rage. It's unknown whether a replacement for Mir has been found.

Sources indicated to MMAjunkie.com that a knee injury is to blame for Mir's withdrawal.

Mir was set to be the first active UFC fighter to cross over to Strikeforce. His matchup with Cormier materialized after fellow ex-UFC champ Tim Sylvia was nixed for a potential headliner with the Strikeforce heavyweight grand prix champion.

In his most recent appearance, Mir replaced Alistair Overeem to fight UFC heavyweight champ Junior Dos Santos at UFC 146 and lost via second-round TKO. It was his second failed bid to regain the undisputed belt after a motorcycle accident forced him to relinquish the title he won at UFC 48.

The broken leg he sustained kept him out of action for nearly two years.

The undefeated Cormier most recently outpointed Josh Barnett to win the Strikeforce heavyweight grand prix after entering the eight-man tournament in the semifinals as an alternate.